1. Field of the Invention
The present invention pertains to apparatus for lifting well fluids from a subterranean reservoir. More specifically, the present invention pertains to differential pressure operated free pistons, sometimes referred to as "gas operated plungers", for lifting well fluids from a subterranean reservoir through a well conduit which extends to the surface.
2. Description of the Prior Art
Differential pressure operated pistons, or plungers, have long been utilized in producing subterranean wells where the natural gas pressure in the well is insufficient to produce a free flow of fluids to the surface. Such devices generally include a free piston or plunger element which slidingly and sealingly engages the inner walls of a well conduit which extends upwardly from the well reservoir to the surface. The well conduit, normally referred to as a production string, may be either a casing string or a tubing string disposed within a casing string.
Typically, the free piston or plunger is provided with external sealing elements for sealingly and slidingly engaging the inner walls of the well conduit. However, the free piston or plunger may typically be provided with an internal passage which bypasses the external sealing elements and allows the piston or plunger to gravitate or fall from the surface of the well to the reservoir. A valve mechanism is typically provided within the body of the piston and is effective to close the internal passage through the piston when the piston has reached a preselected depth in the well. This preselected position may be determined by a stop device, sometimes referred to as a tubing stop, which is semipermanently located in the well conduit. The tubing stop may be relocated, from the surface of the well, at different depths in the conduit as well conditions change.
When the piston or plunger reaches the tubing stop, contact with the stop cause the valve mechanism therein to close, blocking internal passage through the piston or plunger. The piston remains in this position while well fluids accumulate above the piston and gas pressure begins to build. After a pre-selected time, a control valve at the surface of the well opens, allowing flow of fluids to commence. The differential pressure between the surface of the well and higher pressures below the piston force the piston and the well fluids thereabove to the surface. On arrival of the piston at the surface, it is captured in a capturing unit and the internal valve is opened, opening the internal flow passage through the plunger. At the same time the control valve is closed. This allows the piston to gravitate to the bottom of the well conduit until stopped by the tubing stop. The piston valve is again closed and the next cycle begins. Examples of such prior art gas operated or differential gas operated piston or plunger apparatus may be seen in U.S. Pat. Nos. 4,070,134 and 4,923,372.
Most differential pressure operated pistons or plungers of the prior art have inherent problems. One problem is with the seal between the plunger and the inner walls of the well conduit. The seal must be capable of sliding past restrictions within the well conduit; yet slidingly and sealingly engageable with the inner walls of the conduit sufficiently to provide the pressure differential necessary for elevating the piston and well fluids thereabove to the surface of the well. It must be capable of doing so without undue damage to the seal. This type of seal frequently comprises a plurality of metallic pads or longitudinally separated cylindrical segments on the exterior of which are provided relatively smooth cylindrical surfaces. These pads or segments are radially moveable between an innermost position and an outermost position so that the seal may contract as it passes through restrictive portions of the well conduit but may expand to sealingly and slidingly engage the well conduit to provide the necessary seal. Typically, some type of biasing means is provided to bias the sealing elements or pads toward their outermost positions. However a considerable amount of leakage and bypassing occurs between the interior surfaces of the pads or segments and the piston body. This of course increases the pressure necessary to elevate the piston and well fluids from the subterranean formation to the surface of the well. In addition, in the piston designs of the prior art, the segments or pads inherently leak through the longitudinal spaces or separations between adjacent segments or pads. These spaces are wider and leak more as the internal diameter of the well conduit increases, as is frequently the case with upper portions of a well conduit. This type of leakage also increases the pressure necessary for lifting of the piston and well fluids, requiring more piston trips.
Furthermore, the flow and leakage between the interior of the segments or pads and the piston body and between the spaces between the longitudinal separation or spaces between segments or pads causes erosion which will erode the piston body and the sealing elements, further increasing the pressure necessary to elevate the piston and well fluids to the surface of the well and eventually eroding these elements to such an extent that they are no longer useable, requiring repair and or replacement at considerable expense.
In many of the differential pressure operated pistons or plungers of the prior art, particularly those in current use, a rod is concentrically mounted in an internal flow passage therethrough. The purpose of the rod is to reopen the piston valve when it reaches the surface of a well. Typically, the rod would engage a spring loaded stop at the surface of a well causing the rod to move downwardly to engage a valve closure member, unseating the closure member and opening the valve. Such a design severely restricts flow through the internal flow passage as the plunger is falling back to the bottom of the well. In some conditions it restricts the flow to such an extent that the plunger will not fall without added weight. This type of design is costly, requires corrections for the additional moving weight and creates other hazards for the piston as it returns to the surface. With pistons or plungers provided with internal bypasses and valves, fluid flow through the piston (when falling) should be maintained at an uninterrupted maximum. If not, the piston will fall erraticly and may not reach the depth required to close its valve. If the valve does not close the piston will not return to the surface on its own accord, requiring a costly fishing operation to free the piston. In addition to the above mentioned problems, the rod provided to the valve is necessarily small, fragile and susceptible to damage.
Some designs have attempted to solve the problems associated with a rod in the internal flow passage of the piston by providing a rod and a lubricator bumper sub typically installed at the upper end of the well conduit. Such a rod is designed to penetrate the piston flow passage to contact the valve closure member and push it off its seat at the instant the piston contacts a spring loaded stop in the lubricator bumper sub. This requires accurate and sensitive adjustments which are not always possible in field situations. Furthermore, there are many hazards and damage possibilities when the piston, traveling under pressure, is propelled against a stationary rod. There are so many disadvantages of this design, that very few well operators utilize it.
U.S. Pat. No. 5,427,504 discloses a recently designed piston or plunger which eliminates many of the problems of the prior art by providing a sealing assembly which comprises a plurality of longitudinally divided cylindrical segments or pads for sliding and sealing contact with the well conduit walls. Adjacent edges of adjacent ones of the cylindrical segments are provided with overlapping or stepped opposing surfaces which are slidingly engageable with each other as the cylindrical segments move radially between inwardly contracted and outwardly expanded positions. A resilient inner seal is also provided between the piston body and inner surfaces of the segments to reduce leakage between the segments and the piston body. While this tool is much more effective than those of the prior art, the overlapping or stepped cylindrical segments are not easily manufactured. In addition its resilient inner seal of rubber or other resilient compounds will not last for extended periods of time in the harsh environments to which it is subjected.
Thus, the search continues for differential pressure operated pistons or plungers with effective sealing capabilities for lifting well fluids to the surface of a well without the inherent problems of the pistons and plungers of the prior art, i.e. leakage, erosion, erratic operation, unsafe operation, failure, costly remedial operations, etc.